DR4B Tutorial by 333A

It is that time of the season when beginner teams are searching for resources to assist them with building their first DR4B lift, and senior teams are looking for new ways to build taller, stronger, and more stable DR4Bs, improving on their designs from past seasons.

Hence, we decided to make this DR4B Tutorial by 333A.

  • In our guide, you will learn how Double Reverse Four Bar (DR4B) works.
  • New teams will find step-by-step instructions how to build a good DR4B.
  • It explains the main factors of stable DR4B: Screw Joints and Cross Bracing.
  • This guide shares building tricks we’ve acquired and the lessons we’ve learned.
  • Other references are linked for teams to research and improve upon our design.

Finally, we are working on a video counterpart for this guide, which will be found here upon its completion.

Some highlights of the DR4B/8B that we built at the beginning of the Tower Takeover season:

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These are bearing extensions on the rotating bars of the lift. They extend as far as possible while still allowing the screws to go through both sides of the midsection. This minimizes wobble as much as possible:

These are some of the bracing techniques we used, namely X Bracing and K Bracing. They both use triangles to reinforce the lift, since triangles are the only rigid 2D shape. This prevents a lot of torsion that would otherwise become a problem:

This is the rubber band triangle on the bottom linkage of the lift. Using a triangle formation allows the lift to raise with uniform acceleration, which allows for more consistency in a competition. We but them around loose spacers to allow the rubber bands to rotate without the risk of snapping:

Conclusion:

DR4Bs are very useful lifts for VEX Robotics. In addition to being extremely tall, they rise linearly, making them useful for almost any challenge involving lifting objects. It’s clear why they dominated in past games such as In The Zone and Skyrise. However, in order to be competitive, they need to be built with maximum stability.

The most important takeaway from our research was that you should use screw joints whenever possible. This change had the greatest impact on the stability of our lift, and forever changed the way we build anything.

Bracing is also very important, and can’t be overlooked. There are many effective ways of bracing, and all of which have their respective benefits.

Lastly, we learned to not fear the unknown. Had we not tried new things, our lift would still be a twisted wreck that lifted at an angle. Learning more about DR4Bs has not only improved our lifts, but all other aspects of our building.

Remember that there are many great methods for building DR4Bs, not just the one presented in our document. Each one has its own merits, and learning about new methods will help improve your own.

Good luck with your future endeavors!

References
  1. Team 5225A - The Pilons (check out their ITZ season DR4B CADs)
  2. Kepler Electronics Video (great explanations, but axle shaft joints may wobble)
  3. Comparison of VEX joints (spoiler alert: @Owen thinks screw joints are the best)
  4. @technik3k mini-tutorial (on how to build rigid DR4B lift using 2" long screw joints)

Recent DR4B discussion topics:

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DR4B tips from ITZ world champions Team 5225A:

Excerpts from: Tips for Stabilizing a Lift (Prevent Sway)

@technik3k mini-tutorial about building very stable DR4B with 2" long screw joints:

Even more DR4B building tips:
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Rubber Banding Resources:

See the rest of @therealcedz UTRBs post...

Most teams use this sort of rubber band system on their lift (4 bar, 6 bar, 8 bar, etc.):


This system is great due to complexity vs. effectiveness. Just by putting a couple of pieces of metal (in this case, standoffs and screws) and rubber bands, the stress on the motors is greatly reduced.

However, some of you may have come across some flaws with this sort of tensioning system. The rubber bands have a greater upward force when the lift is at the bottom, while the upward force starts to diminish while the lift rises. What this means is that as you lift upwards, the assistive force of the rubber bands start to disappear, as the motors begin to have to do more work. The lift then reaches a point where the rubber bands are completely slack and has no assistive force on the lift.

I created a graph to better illustrate the situation:

Obviously, the numerical values are not accurate as the rubber bands won’t be producing a 100N of force on the lift, but the concept is the same. The Graph also may not even be linear, but the point is that the force diminishes as the lift angle increases.

In addition to the fact that the rubber bands do not work effectively when the lift is raised, high upwards force near the bottom of the lift is also not ideal. With no game pieces in your robot, the lift may start to rise on its own because of the strong upwards force of the rubber bands. One may combat this problem by taking off rubber bands, further reducing the effectiveness of the rubber bands. Others may put a small constant motor value in the code ( while (lift is down) motor value = -10; ) but this is still not ideal.

An ideal graph would look like so:

Constant upwards force regardless of the position of the lift.

My discovery of the uniformly tensioned rubber band system (UTRB for the sake of the phrase’s length) made its way during the 2012 gateway season. While creating a pneumatic lift, I discovered that a UTRB was absolutely necessary due to the fact that pneumatic pistons are quite weak, but also have a constant force along its stroke (save for minimal amount of loss force due to the fact that air gets released every time you activate one). After the robots completion, the robot was able to lift 6 game pieces (3 pounds) to 30” and still be able to bring the lift down with 0 pieces (no weight) all while having a pneumatic lift. To put this in perspective, the pistons could not lift the manipulator up at all without any type of rubber band system. This should demonstrate the effectiveness of the UTRB.

How it works:

Let’s look at the UTRB when one’s lift is all the way at the ground.

If you look carefully, you can see that the rubber bands almost cross the pivot point of the lift.

This means that the force acting on the lift by the rubber bands is very small.
R (crossproduct) F = Torque
Or
R sin(theta) F = Torque.
When theta approaches 180, sin(theta) approaches 0.

Now when the lift is halfway up:

sin(theta) will be greater

When the lift is at the top:

Sin(theta) is the greatest here with a value of 1

Now coupled with the fact that the force of a rubber band decreases as the length decreases and the value of sin(theta) increases as the lift increases, what we get is a(n almost) uniform torque (upwards force acting on the lift).

Here is a video demonstrating the effectiveness of the lift:

https://youtu.be/ertKM1E2ejE

Interestingly enough, during this comp, I noticed that another team had come up with this type of tensioning system. One of 1437’s viewpoint robots had this tensioning system in the back of their robot. (If you look closely enough on the red side, you can see that the back of their robot has a bar jutting out of the back of their lift, which is the same UTRB, but just located in the back).

Now in 2013, John Ma and Kevin Li from WASABI used my tensioning system but in a much simpler way

And here is a video demonstrating their lift:

https://youtu.be/yngtgmTHRRU

The picture, being the more updated version, demonstrates how they only used a couple pieces of metal rather than my original heavy structure (made with a bunch of straight bars and standoffs). Their structure reduces the weight of the structure and I believe this is the simplest it could become. (Use this structure is what im saying basically).

By the way, if anybody is interested, here’s a link of their reveal and pictures of their amazing robots which they go more in-depth to.

1492 WASABI Post-Worlds Reveal

The amount of rubber bands one should use in my opinion should be enough to counter act the weight of the manipulator in addition to some game pieces. For instance, in our gateway robot, when all the air was released from the pneumatics, the lift would start to rise on its own due to the tensioning of the rubber bands. In the case of a motor lift, the idle torque of the motors should be the factor that holds the lift down. In this way, more game pieces could be lifted up (great for high load games like sack attack).

Granted, this tensioning system may not be exactly uniform, as the value of the force provided by the UTRB may waver, but I believe that the deviance is negligible. Some members may do some physics to figure out the exact point to mount the rubber bands to achieve the most uniform tension, but I think it is quite difficult as it requires maybe an upwards of 4-5 variables to solve for.

Heres some more pictures of the UTRB if anyone is interested: http://imgur.com/a/47gnY

Sorry about the quality of the pictures, as they were uploaded to facebook, then uploaded to imgur for the purpose of this guide

Alright! That’s all there is to know about UTRB’s. Thanks for reading, and I hope this helps teams in their quest for awards or yearning for knowledge. If you have any questions, feel free to post below.

Best rubber bands (tensile strength-wise) to use for a lift?

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Where was this in ITZ? XD

Jokes aside, I think that this is a really good resource for new teams that don’t know where to start.

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We didn’t exist during ITZ, and thank you.

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Why didn’t you guys show up to Mundelein? It looks like your bot is complete.

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We decided to scrap that bot. We were trying to make too much happen in too small of a space.

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Sweet baby jesus you madman. I love it.
Do you mind editing your post if people provide good additional resources? This is the most thorough and complete dr4b post I have ever seen, and being that vex has HORRIBLE BASICALLY UNUSABLE FLIMSY FRICTION BASED LINEAR SLIDERS we will never see the simpler linear lifts used in most other robotics, so linkage based lifts are here to stay.

Also, I would like to say that using the new 4-post nut retainers as the base of you screw-shaft is a good trick for getting even more consistent geometry with little added parts. I know that centering screws is a challenge and the shaved down bearings are clever, but the nut retainers are super fast, and do a great job both centering the screw and spreading out the force.

If anyone has programming recommendations, I would like to see a collection of resources for that as well. Programmers may not be aware of how necessary learning pids are when creating a lift. The ability to recall preprogrammed lift heights, make small changes to height without running the motors at 100%, changing the acceleration profile of the drive train based on the height of the lift, or even combining actions like rollers or claws automatically running/opening as the lift stacks or unstacks objectives. Additionally, these pids make autonomous routines so much easier and reliable. You can keep it easy, or look into temperature management, and using the brake function upon decent to help reduce the amount of current used. This is one of the easiest ways to get really creative and have a positive impact on your robots performance without needing to know a ton of math.

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Great tutorial @333A!

I am going to print it and give to every team in our club who is building DR4B or DR6B lifts.

Please, keep up this good work making video tutorials, which I’m looking forward to watch!

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I just realized that the thread title has a typo, @Drow can you fix this?

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this is a great resource wish i had it last year, one thing to add always use C-Channel i tried making a lift with angle bars and they are too wide. you want your lift to fold up small and use as little space as possible. If you look at some of the pictures the clearances are less than a centimeter. that is what you want so you can add stuff like a tray to the lift

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The person who made the thread can change t themselves @333A

No need to message drow

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@Drow can we get this post pinned? It is a shining example of the quality content we want to see more regularly on the forum. Furthermore, if it isn’t stickied, it will inevitably get buried by posts including (but not limited to) people asking for help with their DR4B Lifts.

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@333A I have a few suggestions.

  1. Replace the “1-bar lift” with “2-bar” linkage since the tower counts as one in both the 2 and 3 bar linkages.

  2. I recommend using the phrase “screw-shaft” when describing that assembly since it gives that very important building process a proper name that people can call it. I want builders to learn from this techniques that they can apply elsewhere in their builds, and names are good ways to make people remember things.

  3. Investigate the use of the new nut retainers in construction, as they make much of this build way easier.

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We can’t pin every guide, but we can add it to the wiki…

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It would be nice to have some way of nominating or recognizing quality posts as worthy of putting into the wiki. Maybe a regularly (monthly) occurring wiki-nominating post, closed group, @shoutout, or form.

Or just starting a Best-Of thread for sharing the most informative posts you come across. IDK, what do you all think?

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